FTIR study of the photoinduced processes of plant phytochrome phyA using isotope-labeled bilins and DFT calculations


FTIR study of the photoinduced processes of plant phytochrome phyA using isotope-labeled bilins and DFT calculations

Schwinté, P.; Foerstendorf, H.; Hussain, Z.; Gärtner, W.; Mroginski, M.-A.; Hildebrandt, P.; Siebert, F.

FTIR spectroscopy was employed to analyse the chromophore structure in the parent states Pr and Pfr of plant phytochrome phyA and the respective photoproducts lumi-R and lumi-F. The spectra were obtained from phyA adducts assembled with either uniformly or selectively isotope-labeled phytochromobilin and phycocyanobilin. The interpretation of the experimental spectra is based on the spectra of chromophore models calculated by density functional theory. Global 13C-labelling of the tetrapyrrole allows for the discrimination between chromophore and protein bands in the FTIR difference spectra. All IR difference spectra display a prominent difference band attributable to a stretching mode with large contributions from the methine bridge between the inner pyrrole rings (B-C stretching). Due to mode coupling, frequencies and isotopic shifts of this mode suggest that the Pr chromophore may adopt a distorted ZZZssa geometry with a twisted A-B methine bridge. The transition to lumi-R is associated with only minor changes of the amide I bands indicating limited protein structural changes during the isomerization site of the C-D methine bridge. Major protein structural changes occur upon the transition to Pfr in which the chromophore adopts a ZZEssa-like state. In addition, specific interactions with the protein alter the structure of the B-C methine bridge as concluded from the substantial downshift of the respective stretching mode. These interactions are removed during the photoreaction to lumi-F which affords a ZZZssa-like structure of the chromophore and involves only small protein structural changes.

Keywords: vibrational spectroscopy; chromophore-protein interaction; photoisomerization; isotope- labelling; conformational changes; resonance Raman

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Publ.-Id: 11021